Pistons operating in combustion cylinders commonly have piston ring grooves around the upper, peripheral surface of the piston for receiving piston rings. During the piston's compression and expansion strokes, the ring forms a seal by contacting the cylinder wall and a lower wall of the piston ring groove. A purpose of this seal is to prevent blowby, which is the escape of pressurized fluids from the combustion chamber between the piston and the combustion cylinder wall.
Currently, two types of piston rings are most commonly used in internal combustion engines: rectangular rings and keystone rings. The designations refer to the geometric shape of the ring and groove cross section.
The rectangular ring and groove assemblies have superior capability to control blowby, but in heavy duty applications, such as in heavy duty diesel engines, the rings have a tendency to seize in the grooves due to the carbon residue which is carried by the blowby gases and deposited within the ring grooves. On the other hand, in the keystone ring and groove assemblies the rings have the capability of crushing the carbon deposits and remaining free within the grooves, but have a reduced capability of controlling the blowby. This is due to the fact that the sides of the groove and the ring are both conical and that these two surfaces cannot maintain contact around the entire circumference when the ring centerline is offset relative to the groove centerline. Theoretically, there is only a two point contact which results in a gap between the ring and groove sides around most of the circumference under the offset conditions. Since the piston is offset or tilted within the cylinder most of the time, the resulting increase in blowby is significant. Distortion of the ring under pressure, and wear between the ring side and the groove wall during engine break-in is relied on to establish a seal, but reliance on wear detracts from the advantages that might otherwise result from using highly wear-resistant materials.
Various proposals have been made to improve the blowby controlling capability of keystone piston ring and groove assemblies. Reference is made to U.S. Pat. No. 4,123,072 to Sharpe wherein the piston ring side and the facing piston ring groove wall are shaped to ensure two edges of contact between the wall of the piston ring groove and the facing side of the piston ring. This edge-to-edge or line-to-line structure produces stress at the edges of contact to speed the formation of a seal during an engine's run-in period, but the concentration of stress increases wear on the piston ring side and piston groove wall. Blowby occurs despite two edges of contact since the gap between the edges of the ring side, and the groove wall may permit pressurized fluid to escape. When the piston is eccentric or tilted within an associated cylinder, one edge of the ring may unseat from the groove wall, opening the gap between the ring side and the groove wall through which cylinder pressure may leak. Since pistons are positioned at various degrees of eccentricity or tilt during much of the piston cycle, blowby occurs which impairs engine operation.